We wish to purify and characterize the products of the platelet release reaction and to study their function in thrombosis and cell growth. Using affinity chromatography we have purified human platelet factor 4 to apparent homogeneity and determined the complete amino acid sequence of this first 70 residue protein. We now plan to attempt to demonstrate the cellular synthesis of rabbit platelet factor 4 using short-term megakaryocyte cultures and to seek information on the heparin binding domains of human platelet factor 4 using chemically synthesized peptides of the COOH-terminal region. We plan to purify and characterize the fibroblast growth factor released from platelets during coagulation. Once purified, the proliferative response of cells will be examined with respect to DNA synthesis, cell cycle, and cell density.